This invention relates to glass manufacturing and more particularly to an improved method for manufacturing glass bottles by the blow and blow process.
In recent years, there has been a substantial increase in the number of glass bottles produced annually for use as pressure vessels in the packaging of beer, wine and carbonated beverages. The trend has been toward decreasing the weight of the glass in a given capacity container. This trend increases the importance of uniform glass distribution and maximum material strength. The strength of a glass bottle is limited by the thinnest sidewall area. Therefore, where a glass manufacturing process results in non-uniform wall thicknesses, the walls must be made of such a thickness that the thinnest area will withstand the maximum pressures and stresses placed upon the sidewall or bottom. To provide adequate thickness at the thinnest sidewall or bottom areas, other areas will contain unneeded glass which adds to the weight and cost of the bottle, as well as to the cost of shipping products packaged in the bottle. Therefore, any improvements which increase the uniformity of the sidewalls in a glass bottle used as a pressure vessel reduces both the weight and the material cost of the bottle.
The most common method for forming glass bottles of the "narrow-neck" design is known as the "blow and blow" process. This process is characterized by initially forming a parison from a gob of molten glass and subsequently blowing the parison into a final bottle shape. However, what is commonly known as a "settle-blow wave" occurs in the sidewalls of glass bottles produced by the basic blow and blow process. The bottles also have a horizontal non-uniformity. The sidewalls of the blown bottle have a vertical non-uniformity, being thinner in the region below the settle-blow wave than above the settle-blow wave. The lack of wall thickness uniformity tends toward a weaker bottle and necessitates the providing of extra glass thickness overall in order to create an adequate thickness in the lower sidewall area. U.S. Pat. No. 2,273,777 which issued on Feb. 17, 1942 to W. K. Berthold discloses an improved blow and blow process which reduces the settle-blow wave in the sidewalls of a blown glass bottle. This process involves forming a parison in a two-piece or split blank mold and subsequently blowing the parison to a final shape in a blow mold. The parison is of such a shape that both a blank mold and a neck mold must be of the split type to permit separation of the parison from the mold. The vertical seam separating the mold parts results in non-uniform cooling of the parison which in turn results in a non-uniformity in the wall thickness in a horizontal section of the blown bottle. The parison mold is initially charged with a gob of molten glass and the glass is blown downwardly to fill an annular space defined by the neck mold and a neck pin projecting into the neck mold. Subsequently, the neck pin is retracted and air is blown through the molded neck until the molten glass contacts the walls of the mold cavity and a baffle closing the upper end of the mold cavity. The baffle is removed and the parison is further blown through the molded neck until the molten glass projects a predetermined distance above the blank mold. The split blank mold sections are then separated from the parison and the parison is blown further. After blowing, the completed parison is inverted and inserted into the cavity of a conventional blow mold. Thick side and bottom walls of the pairson quickly reheat the outer surfaces of the parison to a molten condition and the parison sags in the blow mold cavity. Air is then blown through the neck of the sagging parison forcing the molten glass into contact with the walls of the blow mold cavity to complete the blowing of a glass bottle. Although this process reduces the settle-blow wave in the sidewalls of the blown bottle, a noticeable settle-blow wave may still be present. The amount of settle-blow wave present in the bottle is determined by how full the molten glass charge fills the parison mold. Furthermore, the fact that the parison is partially blown in the parison mold with the upper baffle removed permits the surfaces of the end of the parison to be reheated to a less viscous state than the surfaces which remain in contact with the parison mold prior to inserting the parison into the blow mold. This appreciably reduces the strength of the glass in the finished bottle. It has been recognized in the glass industry that increasing the reheat time in the manufacturing cycle after the formation of the parison and before expansion in the blow mold creates a stronger bottle. The Berthold method inherently reduces the reheat time since the parison is formed of relatively thick sidewalls which will rapidly transmit heat to the outer surfaces and since the cool baffle plate is removed from the end of the parison prior to removing the parison body mold sections from the parison. Furthermore, the parison sidewalls will not be uniformly cooled by the split blank mold sections which form the parison. Therefore, the walls of the blown bottle will be non-uniform in a horizontal section.